1. Field of the Invention
The present invention relates to an optical repeater, and more particularly to an optical repeater for use in an optical filter module employed in, for example, an optical access network.
2. Description of the Background Art
In recent years, telecommunications networks have enjoyed wider acceptance with a trend toward a strong demand for access networks having bandwidths of larger communication capacity. Furthermore, a development of networks having bandwidths of larger communication capacity would lead to accepting a variety of service demands from the users, thus possibly rendering traffic on access networks uneven in temporal and spatial distribution. Accordingly, with access networks, a novel network architecture is required to attain efficient network control.
As a network architecture that makes it possible to accomplish optimum communication capacity, currently discussed is a system which has an access network integrated with a metropolitan area network (MAN).
For example, “Metro/Backbone WDM Equipment SpectralWave DW4200 Series”, Product Introduction by NEC Corporation, was searched for on the website, http://www.nec.co.jp/spectralwave/dw4200, on Mar. 24, 2011. The website presented an apparatus having a ROADM (Reconfigurable Optical Add-Drop Multiplexer) function for use in metropolitan area networks. The ROADM function is a reconfigurable multiplexing function of splitting or inserting an optical signal. Use of devices having the ROADM function as components constituting an access network may scale down the network to accomplish a flexible access ring.
Thomas Pfeiffer, “Converged Heterogeneous Optical Metro-Access Networks”, ECOC 2010, Tu.5.B.1, IEEE, 19-23 Sep., 2010, Torino, Italy, describes a metropolitan area access network in which ROADM is integrated with a wavelength division multiplexing (WDM) filter for filtering wavelength division-multiplexed signals.
However, since the controllers described in the above-cited documents are of the active type, they involve the problem that they consume extensive electric power. In order to reduce the power consumption, it is necessary to perform control relying on passive routing.
For example, if a network is built using only WDM filters, then passive routing would be possible. However, the available wavelength band would be limitative. Therefore, in applications requiring a lot of communication channels, there may arise the problem that the wavelength resources would be exhausted.
In order to increase the number of channels, an optical wavelength multiplexing technique could be combined in hybrid with an optical code multiplexing technique. However, if passive optical components are implemented by the fiber Bragg grating (FBG) type of optical encoder and decoder, optical encoded or decoded signals would be carried only by reflected signals as taught in Japanese patent laid-open publication No. 2010-171684. Therefore, in a ring network topology as shown in FIG. 2A or a cascade network topology as shown in FIG. 2B, described later, a combination or division of optical waves may involve the problem that the passive optical components would be complicated in structure.